Generally, a vibration motor is employed inside a mobile communication terminal such as a cellular phone to notify a user of an incoming call. A coin-shaped flat vibration motor, greatly reduced in size and thickness, is widely used to meet the requirements of smaller size, lighter weight and lower power consumption in the mobile communication terminal.
The flat vibration motor comprises mainly a permanent magnet as a fixed member and a rotor as a rotation member, and an electric connection between the power supply (+, −) and the rotor is made generally by brushes and a commutator.
In the early days, vibration was attained using an eccentric plate mounted on the output axis of the rotation motor, or a fan-shaped rotor made eccentric as a whole toward its one side and having three armature coils mounted thereon with no overlapped portion therebetween.
Such a fan-shaped vibration motor using the fan-shaped rotor includes an eccentric rotor having a plurality of armature coils spatially biased toward one side, an axial rod mounted in the center of the eccentric rotor, and an axial bearing and a case, both for supporting the eccentric rotor so that it is freely rotated with the help of the axial rod and the bearing. The fan-shaped vibration motor also includes a housing made of a bracket, a donut-shaped permanent magnet mounted on the bracket for providing magnetic flux to the eccentric rotor, brushes disposed inside the permanent magnet, a brush base inserted therein, and a commutator disposed on one surface of the eccentric rotor so as to be in sliding contact with the top of the brushes.
The vibration motor has such a structure that the eccentric rotor having the plurality of armature coils spatially biased toward one side is rotated; the permanent magnet is fixed; and the brush-type motor that receives the drive power through the brushes and commutator is adopted, thereby leading to the following problems.
That is, such problems are widely known that, because the vibration motor is small in size with a diameter around 10 mm, the brushes and commutator are weak in their structure and low in durability, in result, shortening the life span; the cost is increased due to difficulty in manufacturing the brushes and commutator; and spark and noise caused by the connection structure exerts a bad influence upon the operation of other various electronic devices.
Because other conventional vibration motors used in the mobile communication terminal, etc., also use brush-type motors each having an eccentric unbalancing weight without exceptions, they have the same problems. Only a noise filter, etc., is currently available to resolve these problems.
In addition, since the conventional vibration motors have large air gaps between the rotor coils that cause a relatively large amount of magnetic flux leakage, there are problems that the vibration motor is lowered in efficiency, and increased in power consumption, consequently reducing the lifetime of the mobile communication terminals.